Strain gauges that rely upon a change in an electrical characteristic, such as a change in resistance or capacitance, in response to applied strain are well known and have been widely employed for many years past. Because such strain gauges require electrical power for their operation, those gauges present a hazard in environments where electricity can cause an explosion or a fire or cause an electrical shock to personnel. Further, the presence of electrical power can interfere with the performance of sensitive equipment and the performance of the electrical strain gauge itself may be adversely affected by electromagnetic interference generated by other apparatus in its vicinity. In addition, the electrical current flowing in the gauge generates heat which tends to affect the performance of the gauge. In the terminology of the electrical engineer, the performance of the conventional electrical strain gauge can be adversely affected by EMI (electromagnetic interference), the gauge may be adversely affected by heat genterated by current flow in the gauge, and the gauge itself may be a source of "noise" (i.e. electromagnetic radiation) for other electrical and electronic apparatus. Electromagnetic interference, for example severely limits the use of electrical strain gauges in or near electric arc welding equipment. Consequently, efforts have been made to develop non-electrical strain gauges for use in high EMI environments and for use in those environments where the presence of electricity is dangerous.